In the field of therapeutics, the use of syringes for injecting fluids into tissues or natural cavities in the body is extremely widespread. A syringe is known to be composed essentially of a cylindrical barrel in which a plunger slides, a body whose base bears a nozzle of a suitable shape to which can be fitted a hollow needle, for example a needle made of steel or nickel, the barrel thus forming a reservoir for the fluid to be injected. The structure or composition of the plunger can vary according to the model. It is, in any case, provided at one of its ends with a joint to ensure sealing with the barrel of the syringe and, at its other end, which is always external to the barrel of the syringe, a plunger head to facilitate its handling. This classical syringe makes it possible to perform in the habitual manner the operations required for an injection as commonly practised, that is in the first place, detaching the joint at the bottom of the syringe and then, by applying traction to the plunger to extract it from the barrel of the syringe, drawing in a certain quantity of fluid. Then, with the syringe in inverted position, that is with the needle pointing upwards, a slight pressure on the plunger causes any air remaining in the reservoir to be discharged, this operation possibly being followed by the drawing in of a complementary quantity of fluid and again the essential discharge of the air.
Before injection properly speaking, slight re-aspiration is practised after insertion for checking purposes.
There are known disposable syringes used for vaccinations which are pre-filled in the laboratory and with which it is no longer possible to draw in fluid after the injection. However, as most syringes used are not pre-filled and since it has to be possible for the to and fro movements of the plunger to be performed for the operations described above, or for manipulations in an empty condition, it is not possible to adopt this system.